docs: def build_arbitrary_mfd(mfd):
      """
      Parses the arbitrary MFD as a Node
  
      :param mfd:
          MFD as instance of :class:
          `openquake.hazardlib.mfd.arbitrary.ArbitraryMFD`
      :returns:
          Instance of :class:`openquake.baselib.node.Node`
      """
      magnitudes = Node("magnitudes", text=mfd.magnitudes)
      occur_rates = Node("occurRates", text=mfd.occurrence_rates)
      return Node("arbitraryMFD", nodes=[magnitudes, occur_rates])